The present invention relates to an interface control system located between a personal computer and a CD-ROM driver, and more particularly, to an interface control system of a memory mapped input/output (I/O) method using an Industry Standard Architecture (ISA) bus.
Recently, since the field of multi-media personal computers (hereinafter referred to as "MPCs") has newly developed a compact disk read only memory (CD-ROM) unit has been widely used as a media to store a large quantity of information. CD-ROM units can be divided into high-priced and low-priced models. A high-priced CD-ROM model adopts the Small Computer System Interface (SCSI) system which is an industrial standard for interfacing with a personal computer (PC) for input and output, and a low-priced CD-ROM model adopts a unique interfacing method using a PC-BUS (which is also called an ISA BUS). The bus is a channel connecting various devices and is specified as a series bus or parallel bus according to the composition and number of signal lines. Also, a bus can be a standard bus (used commonly) or an exclusive bus (used only for a specific system).
The SCSI system is for high-priced CD-ROMs and is a standard bus which originated from the Shugart Associates Standard Interface (SASI) manufactured in 1979 by Shugart which is a manufacturer of floppy disk drivers (FDD) and hard disk drivers (HDD). Then, SASI was renamed as SCSI and revision 15 thereof was recommended by the American National Standard Institute (ANSI) in 1982. SCSI revision 17B was completed in 1985 and notified publicly as the ANSI standard in 1986. SCSI signal lines are composed of nine control signals, eight data lines, data parity bit lines and power source ground lines. As is well known, the SCSI bus is characterized in that it has a distributed bus arbitrating function to which priority order is allotted and that it can accommodate a large quantity of high-speed peripheral equipment. However, the manufacturing process of such an interface control system for CD-ROM drivers using a SCSI bus is an expensive task.
Meanwhile, ISA is an interfacing method using an extension slot used for IBM compatible personal computers. The ISA is characterized in that all personal computers exceeding an XT level are basically equipped with a 62-pin edge connector and all personal computers exceeding an AT level are equipped with eight direct memory access (DMA) channels, sixteen interrupt channels, a 16-bit data bus, an address bus extended to 24 bits and an additional 36-pin edge connector. The ISA interface is constructed so that an ISA pin signal and a signal of the subject system to be interfaced are interfaced using logic elements without passing through another common interface system. There are three methods for reading or writing data through the utilization of such an ISA bus: direct memory access, memory mapped I/O and I/O mapped I/O.
In the DMA method, the central processing unit (CPU) of a computer does not perform input and output operations directly, but DMA chips which are dedicated chips for input and output purposes, request the central processing unit (CPU) to offer the bus to access a memory directly without passing through the CPU, when input and output operations are necessary. The memory mapped I/O method uses the same address space in allotting addresses of input and output devices as shown in FIG. 1A, and does not provide separately a memory space and I/O space. Hence, the control signal for reading or writing from/to a memory space is also used as the control signal for reading or writing from/to the input/output space. The I/O mapped I/O method divides the memory space and input/output device space into the respective address spaces as shown in FIG. 1B and requires respective control signals for reading and writing from/to the memory space and input/output space. Since an MPC limits the bandwidth of the CD-ROM driver to 40% that of a CPU, in order to keep a real time transfer rate of 150K bytes per second by eight bits within such a limited bandwidth, the memory mapped I/O method should be used.